In enzymology, a mandelate 4-monooxygenase (EC 1.14.16.6) is an enzyme that catalyzes the chemical reaction

(S)-2-hydroxy-2-phenylacetate + tetrahydrobiopterin + O₂ ${\displaystyle \rightleftharpoons }$ (S)-4-hydroxymandelate + dihydrobiopterin + H₂O

The 3 substrates of this enzyme are (S)-2-hydroxy-2-phenylacetate, tetrahydrobiopterin, and O₂, whereas its 3 products are (S)-4-hydroxymandelate, dihydrobiopterin, and H₂O.

This enzyme belongs to the family of oxidoreductases, specifically those acting on paired donors, with O2 as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derived from O2 with reduced pteridine as one donor, and incorporation of one ato of oxygen into the other donor. The systematic name of this enzyme class is (S)-2-hydroxy-2-phenylacetate,tetrahydrobiopterin:oxygen oxidoreductase (4-hydroxylating). Other names in common use include L-mandelate 4-hydroxylase, and mandelic acid 4-hydroxylase. It employs one cofactor, iron.